What factors influence and determine the magnitude of ionization energy for an element?

For ionization energy, we measure the energy change for the following process:

#"Element"(g) + "energy"rarr["Element"]^+(g) + e^-#

Incomplete electron shells shield nuclear charge very ineffectively. This is especially true for #d# and #f# orbitals, which are multilobed and do not have uniform distribution. As protons, fundamental, positively charged, massive nuclear particles, add to the nucleus, #Z# increases, and nuclear charge increases disproportionately. The result is that more energy must be pumped into the above equation to remove an electron, and to ionize an element.

Once an electron shell is full, the electrons start building a new shell, and the whole process starts again in a new Period. This process is also manifest by the decreasing atomic size observed across the Period from left to right. Rightmost atoms are smaller because of increased #Z#. I urge you to look at the data in your text.

Ionisation energy is the energy required to remove an electron from outermost shell of an atom ( state of element must be gaseous )
REASON_As removal of electron if element is in solid or liquid is very difficult
This goes on increasing as we move from left to right in a period
REASON- as we move from left to right the nuclear charge goes on increasing and hence the hold of nucleus on the outermost shell of an atom increases and thus it becomes difficult to pull them out
Thus more energy is needed to remove them out
Ionisation energy is ENdothermic in nature i.e. energy is supplied

1.for inert gases Have the highest ionisation energy (inert gases are those which have completely filled orbits ) inert gases in the periodic table is the 18th group

2 .Conclusion as the increase in the nuclear charge the size decreases and hence it becomes difficult to get the electrons out of the outermost shell
The increase in nuclear charge also decreases the size of the elements (just for information)